We have a deep well pump & just put on a new pressure switch.
The switch turns the pump on at 20 lbs and off at 40, the way it says it should.
In about two minutes the pressure drops and kicks the pump on until it reaches 40 again. It keeps this up. If we shut the valve on the water line going to the house from the tank, the pressure stays up. We checked all water appliances in the house and there aren't any leaks. Does that mean we have a lead in the water line to the house? If so, do we have to dig up the whole line to find it? It's about 100 ft. from the house.
Thank you,
Paul

Hi Paul and welcome to The Plumbing Page at AskMeHelpDesk.com
Before we get to thinking the worst let's nail down the source of the loss. I realize you've checked these but humor me, Run the pump up and shut the house shut off valve, Stop the pump and watch the gage, If it's still falling then leave the house shut off and shut off the service line from the pump to the house. Gage still falling? If so you have just located your problem. You haven a faulty check/foot valve that must be replaced, Good luck. Tom